ESR methods were used to detect cupric ion on pulmonary arterial endothelial cells grown to confluence on gelatin-coated Biosilon beads (see highlight). Upon addition of 150 micro molar Cu(II) to these cells, EPR spectra were obtained from cells and supernate after various times of incubation with Cu(II) and in the presence and absence of bathocuprione, a colorimetric agent for Cu(I). An EPR signal for a type 2 Cu(II) site with g(parallel)=2.18 and A(parallel)=180G is attributed to cupric ion bound to cells. Addition of bathocuprione not only formed a complex with Cu(I), but also with Cu(II) with g(parallel)=2.19 and A(parallel)=160G. Additional studies are planned to determine the number of nitrogens bound to the Cu(II) on cells using low frequency, S-band, EPR methods. Further work is planned to help understand the significance of this site with respect to trans-membrane electron transport.